1. Field of the Invention
The present invention relates to a rewritable optical disc formatted for recording signals both to a recessed recording track formed by means of a guide groove and to an unrecessed recording track formed between said guide groove, and to a driving apparatus therefor.
2. Description of Related Technology
A land and groove recording method whereby data is recorded both to the recessed part (the "groove") of a normally spiralling guide channel and to the unrecessed media area between the groove spirals (the "land") of the guide channel has been proposed as the recording method of a high capacity rewritable optical disc as a means of improving the recording density. This method obviously has the effect of greatly improving the recording density because the track pitch of the recording tracks can thus be halved on a disc of a given groove pitch.
A conventional optical disc formatted for land-groove recording is described in Japan Unexamined Patent Publication (kokai) S63-57859 (1988-57859) and shown in FIG. 13 by way of example only. As shown in FIG. 13, both groove 94 and land 95 are formed by means a guide channel cut into the surface of a disc substrate, and a recording layer 91 is then formed over the entire disc surface. Recording pits 92 are formed in the recording layer of both the groove 94 and land 95. The groove 94 and the land 95 each form a continuous recording track on the disc. Data recording and reproducing are accomplished with this optical disc by scanning the groove recording track or the land recording track with the focused laser beam spot 93 of the optical disc drive device. It should be noted that with this conventional land-groove recording track format the guide channel is formed as a single continuous spiral channel on the disc. The groove 94 and land 95 recording tracks are thus connected with each forming a single continuous spiral recording track, respectively.
Note that this disc format is referred to as a "double spiral land-groove format" or DS-L/G format in order to distinguish this disc format from the "single spiral land-groove format" or SS-L/G format described next below.
The single spiral land-groove format is shown in FIG. 14. In this format a single spiral is separated into plural groove recording tracks each equivalent to one circumference of the disc and plural land recording tracks each equivalent to one circumference of the disc where the land tracks are disposed between the groove tracks and the groove tracks and land tracks alternate from the beginning to the end of a single spiral formed on the disc. An optical disc formatted with a single spiral recording track with alternating contiguous groove recording tracks and land recording tracks as shown in FIG. 14 is described, for example, in Japan Unexamined Patent Publication (kokai) H4-38633 (1992-38633) and Japan Unexamined Patent Publication (kokai) H6-274896 (1994-274896).
A particular advantage of this single spiral land-groove format is that it is well-suited to continuous data recording and reproduction because the recording track is a single contiguous track on disc. This is particularly important, for example, in video applications because continuous data recording and reproduction is essential to moving picture reproduction. With a conventional double spiral land-groove format as shown in FIG. 13, however, the land track and groove track are formed as discrete recording spirals as described above. To shift and continue recording or reproduction from the land track to the groove track, for example, it is therefore necessary to interrupt recording or reproduction at at least one place on the disc surface in order to access the groove track from the land track. The same is true when continuing recording or reproduction from a groove track to a land track. While it is possible to avoid this interruption in recording or reproduction by, for example, providing a buffer memory, this results in increased cost. Note that this increased cost is avoided with the single spiral land-groove format described above.
As described in Japan Unexamined Patent Publication (kokai) H6-290465 (1994-290465) and Japan Unexamined Patent Publication (kokai) H7-57302 (1995-57302), the transition point between alternating groove tracks and land tracks is detected in a SS-L/G format disc, and tracking servo polarity is changed at the detected transition point to track either the groove recording track or the land recording track.
The method of forming the pre-embossed pits of the address signal proposed for an optical disc using the conventional land-groove recording track format is described next below. There are three known methods of forming the pre-embossed address signal pits in the conventional double spiral land-groove format as shown in FIG. 15.
With the independent land/groove addressing method shown in FIG. 15A, a unique sector address is assigned to the land track sectors and groove track sectors. If the width of the address signal pits is equal to the groove width, the pits will be connected to the pits forming the sector address signal of the adjacent track, and address signal detection will not be possible. The address signal pit width must therefore be less than the groove width, and is usually approximately half the groove width.
However, if grooves and pits of different widths are to be continuously formed during production of the optical disc master, it is necessary to change the diameter of the laser beam when cutting the pre-embossed pits and when cutting the grooves. It is therefore necessary to cut the master disc using two laser beams, one for cutting the grooves and one for cutting the pits. This requires high precision alignment of the beam spot centers because an offset between the two beam spot centers causes an offset between beam tracking when reproducing the address signal pits and when recording and reproducing user data. This degrades the quality of the reproduced data. More specifically, a tracking offset increases the error rate, which leads to lower data signal reliability. It is therefore necessary to precisely align the two laser beams, which leads to increased cost during master disc production.
Considering these problems, a method of cutting both grooves and pits using a single laser beam so that the address signal pit width is substantially equal to the groove width as shown in FIG. 15B and FIG. 15C is preferable in terms of the precision and cost of disc production.
The format shown in FIG. 15B is that of a conventional optical disc as described in Japan Unexamined Patent Publication (kokai) H6-176404 (1994-176404). This format uses a common land/groove addressing scheme. The pre-embossed pits of the address signal are formed at approximately the center of a land track and groove track pair such that both tracks can be addressed using the same single address signal pits.
The format shown in FIG. 15C is that of a conventional optical disc as described in Japan Unexamined Patent Publication (kokai) H7-110944 (1995-110944). This format uses an independent land and groove addressing scheme in which an independent address is assigned to each land track and each groove track using pre-embossed pits formed offset in a line parallel to the track such that the address signal pits for adjacent tracks will not overlap.
In addition to the track format and sector format considerations of the land and groove recording methods described above, it is also necessary to consider servo characteristics.
A single beam optical system is used with recordable optical discs as a means of improving the light utilization efficiency. In the push-pull method, which is one example of such systems, a sensor offset occurs as the lens shifts in the radial direction. A tracking offset can lead to crosstalk and cross-erase problems, and is therefore a significant problem in high density recording. It is therefore necessary to apply offset correction to eliminate any tracking offset. Various offset correction methods have been studied.
Using the conventional methods of inserting address signals for land and groove recording, it has not been possible to achieve the characteristics required to achieve the tracking offset correction needed with single spiral land-groove format optical discs. With the common land/groove addressing method shown in FIG. 15B, for example, the pits are formed on only one side and the tracking offset tends to simply increase during address signal reproduction. Furthermore, not only does this also happen with the discrete land and groove addressing method shown in FIG. 15C, but tracking offset detection is also difficult.
A typical conventional method of compensating for the tracking offset that occurs in a push-pull tracking servo method is described in Japan Examined Patent Publication (kokoku) H7-46430 (1995-46430) and known as the so-called "composite track wobbling" method. This method is described as continuously applying push-pull tracking servo control to "an optical disc wherein a header area comprising a pit sequence offset laterally from a track center disposed to a particular place, and a data recording area comprising a preformed groove of a particular depth, are formed alternately along a predetermined track." Using the symmetry of the signal amplitude when the wobbling pit sequence is reproduced in the header area, the tracking servo is controlled so that the signal amplitude reproduced from the pit sequence wobbling laterally to the track center is the same on both sides of the track center to compensate for low frequency tracking offset.
This technique is more effective than those shown in FIG. 15 with respect to inserting the header address signal.
Technologies related to the sector format of rewritable optical discs are described below.
One example of the sector format of a rewritable optical disc is that of an ISO-standard 130 mm magneto-optical disc with a double-sided recording capacity of approximately two gigabytes (2 GB). This sector format is standardized in ISO-13842, "Information Technology--Extended Capacity Rewritable and Read-Only 130 mm Optical Disk Cartridges." The sector format of a sector with a 512-byte user data area is shown by way of example in FIG. 16.
In this example each sector comprises a header area containing address information and a data recording area. The header area is formed on the land from preformatted embossed pits that are narrower than the land, and the data recording area is formed on the land. Each recording sector is 799 bytes long, including a 512-byte user data area.
The header area comprises in sequence from the beginning thereof a sector mark SM that is used for detecting the beginning of the sector and consists of a mirror surface and embossed pit of a length that does not occur in the data modulation signal, a single frequency pattern area VFO1 for reproduction clock synchronization, address mark area AM for byte synchronization during header reproduction, and address area Pid1 for storing the sector address information. This sequence of single frequency pattern area VFO2 for reproduction clock synchronization, address mark area AM for byte synchronization during header reproduction, and address area Pid2 for storing the sector address information is then repeated, and the preformatted header then concludes with a postamble area PA for completing modulation.
The lengths of these areas in the header are sector mark SM, 8 bytes; VFO1, 26 bytes; address mark AM, 1 byte; Pid1, 5 bytes; VFO2, 20 bytes; address mark AM, 1 byte; Pid2, 5 bytes; and postamble PA, 1 byte. Note that the first VFO1 of the two single frequency pattern areas VFO in the header area is longer than the second VFO2.
The physical address area Pid comprises 3 bytes containing the sector address information and Pid number, and a 2-byte address error detection code. The sector address is calculated based on the track address written to bytes 1 and 2, and the sector address written to the low six bits of byte 3.
The data recording area comprises in sequence from the beginning thereof a laser power adjustment area ALPC with an adjustment margin Gap therebefore and after, a single frequency pattern area VFO3 for synchronizing the reproduction clock of the recorded data, a synchronization mark Sync for byte synchronization during reproduction, a data area Data, and a buffer zone Buffer for absorbing variations in disc rotation and clock frequency. Note that the data area Data contains the user data written to the sector, a CRC for error detection and correction, and a resynchronization byte Resync for recovering from synchronization loss.
The lengths of these areas in the data recording area are 10 bytes for the ALPC and Gap areas; 27 bytes for VFO3; 4 bytes for Sync; 670 bytes in the Data area; and a 21-byte Buffer. Note that VFO3 is longer than VFO1 in the header.
Note, further, that (1,7) modulation code is used in this standard where the coding parameters of the modulated signal expressed in the format (d,k;m,n) are (1,7;2,3). In (1,7) modulation code the shortest mark length Tmin is (d+1)T, which equals 2T, and the longest mark length Tmax is (k+1)T, which equals 8T. A 2Tpattern, which is the pattern with the shortest period in (1,7) modulation coding, is used for VFO1, VFO2, and VFO3. The modulated channel bits are recorded using the edges of the recording marks in a NRZI format so that data is expressed by the leading and trailing edges of each recording mark on the disc. It should be noted here that this is the recording method used in the invention described in this specification.
It should be further noted that a land and groove recording method has at present not been achieved using either a double spiral or single spiral recording track, and a physical format such as the sector format of a conventional magneto-optical disc has also not been achieved.
It is also necessary to consider compatibility with the format used in read-only optical discs in digital video applications when devising the sector format for a rewritable optical disc. For example, if a sector format providing the greatest possible compatibility with read-only digital video discs (DVD) comprising 26 synchronous frames of 93 bytes each in one sector, i.e., 2418 bytes/sector, is to be achieved, it is at least necessary for each sector in the rewritable optical disc to be formatted with the ability to store 2418 bytes of user data in the data area with the sector length, including the header area, being an integer multiple of 93 bytes.
One problem with conventional optical disc media using a land-groove recording track format when the optical system of the recording and reproduction apparatus uses a single beam for tracking is that an offset occurs in the tracking sensor accompanying a shift in the objective lens. This makes it difficult to detect the tracking offset using the recorded address signal.
While it is necessary to detect with high reliability the transition point between the land track and groove track in an optical disc using the single spiral land-groove format, such high reliability detection is difficult using the conventional methods of recording the address signal to a land-groove recording track format optical disc.
As with read-only optical disc media, rewritable optical disc media are also used for video applications. In order for a reproduction device to be able to reproduce both types of optical disc media at the lowest possible cost, it is therefore necessary for the format of rewritable optical disc media to be compatible with the format of read-only optical disc media so that the greatest possible number of common reproduction circuit components can be used in the reproduction device.
It is therefore necessary when adding address information to the physical disc format to ensure the read reliability of the address information and the length of the data recording area required in a phase-change medium.